A Pilot Study of the Value of 18F-Fluoro-Deoxy-Thymidine PET/CT in Predicting Viable Lymphoma in Residual 18F-FDG Avid Masses After Completion of Therapy

BACKGROUNDDespite its success in diagnosing and staging lymphoma, F-FDG PET/CT can be falsely positive in areas of posttreatment inflammation. 3′-F-fluoro-3′-deoxy-l-thymidine (F-FLT) is a structural analog of the DNA constituent thymidine; its uptake correlates with cellular proliferation. This pilot study evaluates the ability of F-FLT PET/CT to distinguish viable lymphoma from posttreatment inflammatory changes in F-FDG avid residual masses. METHODSTwenty-one patients with lymphoma with at least 1 F-FDG avid residual mass after therapy underwent F-FLT PET/CT imaging. F-FDG and F-FLT uptake values were compared, including quantitative pharmacokinetic parameters extracted from the F-FLT time activity curves generated from dynamic data using graphical and nonlinear compartmental modeling. RESULTSThe true nature of the residual mass was confirmed by biopsy in 12 patients (8 positive and 4 negative for viable lymphoma and by follow-up CT and/or repeat F-FDG PET/CT imaging over 1 year); among the remaining 9 patients, 7 lesions resolved or decreased and 2 showed growth indicative of lymphoma. F-FLT PET SUVest.max was significantly higher in tumors than in benign lesions (5.5 [2.2] vs 1.7 [0.6]; P < 0.0001), whereas the difference in F-FDG SUVs was not significant (malignant, 7.8 [3.8] vs benign, 5.4 [2.4]; P = 0.11). All of the benign lesions had an F-FLT SUVest.max of less than 3.0. CONCLUSIONSF-FLT shows improved specificity over F-FDG in distinguishing residual lymphoma from posttreatment inflammation and may be useful in the evaluation of patients with residual F-FDG–positive masses after completing therapy.